1. Technical Field
This invention relates generally to communications and in particular to a system and method for line probing and provisioning.
2. Related Art
Historically, new communication technologies are continually being introduced to improve the ease and rate at which data can be exchanged between remote locations. One factor that must be considered when communicating electronic data is the medium over which the data will travel. This is often referred to as determining the channel quality, line characteristics, line transfer function, insertion loss, or channel impulse response. One example medium that is commonly installed throughout the world is twisted pair conductors as were traditionally used to provide telephone service between a central office telephone facility and a residence or business.
The medium must be considered because the medium can affect the rate at which communication may occur. For example, digital subscriber line (DSL) technology utilizes twisted pair conductors. The rate at which systems using the DSL standards may operate is determined in part by the electrical characteristics of the twisted pair between a transmitting device and the receiving device. The factors that control the rate of communication may include the distance between the receiver and transmitter, presence of bridge taps or load coils, the quality of the twisted pair, the quality of connections to the twisted pair, and the amount of noise that the twisted pair picks up, such as cross talk noise. Moreover, the line may have certain unique frequency ranges at which communication should not occur due to noise or other undesirable line attributes.
It is often desirable to determine the characteristics of the line prior to communicating data so that a maximum data rate may be determined. This improves reliability and allows the receiver/transmitter pair to communicate at a data rate that the line can support. A lower data transmission rate than the maximum data rate may be used for operational reasons.
Various methods have been proposed to measure the line characteristics. One method comprises sending a first signal at a first frequency on the line and analyzing the response of the tone to the effects of the line. Based on the response of the first signal, a second signal at a second frequency is sent and the effect of the line on the signal is evaluated. This process continues in a searching or hunting manner to determine the characteristics of the line or until an acceptable data rate is determined after a fixed number of iterations.
An alternate method comprises sending pseudo-random signals on the line, which are of known bit patterns, and then calculating the bit error rate of the received signal via comparison techniques. If the bit error rate is too high, another pseudo-random signal is sent at a different data rate and the calculation process repeated. This process is reiterated until an acceptable bit error rate is calculated.
While these methods are often implemented in prior art devices, they are plagued by several drawbacks. One drawback is that these methods of line evaluation are very time consuming. Both methods implement a search method that performs several reiterations in serial order in an effort to arrive at a line characterization. Each reiteration takes a certain amount of time and hence an undesirably large period of time is consumed when numerous reiterations are performed. In the particular case of the BER method, the time required to compute the BER can be quite lengthy. Numerous iterations may be required since an error probability of less than 1e-7 is desired. As a result, a new communication technology designed to speed the rate of communication may appear to the user to take longer to configure and initiate operation than the prior xe2x80x98slowerxe2x80x99 technology.
Another drawback of prior art methods is that because of their hunt-and-peck type operation, the line characterization may be inaccurate. For example, the search process using different tones or data rates does not perform an analysis at each frequency in the spectrum. Hence, the prior art method of line characterization may not provide all the information necessary to fully characterize the line and thereby not identify undesirable frequency bands. In addition, the prior art methods propose the use of data rates that in many instances were so inaccurate that the line could not support the proposed data rate. This occurred because the processing of the probe signals did not correctly characterize the line. Moreover, if the line characterization was inaccurate, the communication session may fail during use and the time consuming line characterization process would have to be repeated. Even if repeated, the line characterization may again be inaccurate and the communication session will again fail.
The invention overcomes the disadvantages of the prior art by providing a signal for line probing and a method for processing the line probe signal that provides for more rapid and efficient line probing.
In one embodiment the invention comprises a line probe signal and method of generating the same for use in determining line characteristics. In one embodiment the invention comprises a method and apparatus for processing a line probe signal to determine channel characteristics, such as to determine a data rate for the line.
One embodiment of the invention comprises generation and/or use of sequence signals for line probing. The inventors discovered that sequence signals provide advantages for use as line probe signals as compared to prior art signals, methods, or apparatus used for line probing. One example type of sequence signal comprises a signal with good autocorrelation properties. Good autocorrelation properties provide the advantage during processing of approximation to an impulse signal to thereby estimate the channel response and reveal the line characteristics. Autocorrelation is a type of correlation.
In one embodiment an example signal comprises a maximal length sequence (M-sequence). These sequences may be selected with various periods. Sequences with long periods provide a more accurate and complete impulse response based on more probing tones and provide the advantage of noise spreading during signal processing. Sequences with a short period provide the advantage of faster line probing.
In one method of operation, a sequence signal is generated using a scrambler device of a transmitter portion of a transceiver. The transmitter sends the sequence signal over the line for purposes of line probing. The line probe signal may be transmitted at various times or at start-up of the transmitter. After transmission, a receiver receives the sequence signal and performs processing on the sequence signal to evaluate the line characteristics. Based on the calculated line characteristics, a desired or maximum data rate for the line may be determined. Various methods exist for processing the sequence signal and hence the inventive use of sequence signals with good autocorrelation properties is not limited to a particular method of processing or generation.
In one embodiment of the invention a method and apparatus for processing the line probing sequence is provided. Processing the line probe sequence signal is performed to evaluate the noise on the line and/or to calculate and determine line characteristics so that in one embodiment a desired data transmission rate may be calculated. In one instance, the desired data rate is the maximum data transmission rate that the line will support.
In one embodiment and method of operation a received sequence signal is processed by performing a cross correlation operation on the received signal and a reference signal to generate an output signal generally approximating an impulse signal transmitted over the line. The line characteristics may then be determined based on the estimated impulse response.
In one embodiment and method of operation noise on the line may be evaluated by ceasing transmission of any signal by the transmitter and monitoring, at the receiver, for any signal received. Because the transmitter is not sending any signal, any received signal is considered to comprise noise. One example of noise comprises crosstalk.
In one embodiment, the data rate may be determined by establishing a target signal to noise ratio (SNR) that is acceptable. Coding gain and a desired margin may be considered in the SNR target selection. After a target SNR value is selected, one method of operation calculates the highest data rate where the computed SNR is greater than the target SNR value. This calculation may be based on the line noise in the frequency domain and the received signal power spectral density, which is a function of the impulse response of the line. Interpolation around the calculated point may optionally be performed to refine the data rate estimation.
The various embodiments, features, elements, and aspects of the invention as described broadly herein may be combined in any combination or may be implemented individually. For example, an embodiment comprising use of a sequence signal for line probing may be implemented alone or in conjunction with a method of processing the sequence signal to obtain a desired or maximum data rate for the line being probed. Thus, the scope is not limited to only the described combinations but is intended to cover any various combination as might be contemplated after reading the specification and claims.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages are included within this description, are within the scope of the invention, and are protected by the accompanying claims.